1. Field of the Invention
This invention relates to a process for preparing a polyurethane foam by reacting a polyol having at least two active hydrogen atoms with a polyisocyanate, in the presence of a potassium .beta.-tertiary-aminopropionate as a catalyst, whereby good foams can be obtained in a broad range of the index of the mole ratio of the isocyanate groups to the active hydrogen atoms.
2. Description of the Prior Art
According to conventional methods, polyurethane foams are prepared by reacting a polyester-type or polyether-type polyol having at least two active hydrogen atoms, with a polyisocyanate, in the presence of water or a foaming agent such as Freons, a foam-regulating agent such as a silicone and another surface active agent (which is not used in some cases), a catalyst, a filler, a flame retardant and the like. As the catalyst, there are generally employed tin compounds such as stannous octanoate and dibutyl tin laurate and tertiary amines. In the art of preparing polyurethane foams it is known that a polyurethane foam having an isocyanurate structure can be prepared if the amount of the polyisocyanate present during the reaction is such that the amount of isocyanate groups is in excess in relation to the amount of the active hydrogen atoms of the polyol. As the catalyst used for this reaction, there are generally employed alkali metal carboxylates such as potassium acetate and sodium acetate, non-basic metal salts such as lead octanoate and lead naphthenate, and tertiary amines such as dialkylaminoalkylphenols.
However, the activity of all of these known catalysts is greatly influenced by the index, i.e., the coefficient indicating the molar ratio (.times. 100) of the isocyanate groups to the active hydrogen atoms, an index value of 100 indicating the equivalent amount and an index value of 200 indicating that the amount of the isocyanate groups is twice the equivalent amount. For example, when a tertiary amine or tin compound is used as a catalyst, good foams cannot be obtained unless the catalyst is employed with an index of 100 to 105. Further, in the case of catalysts for preparing polyurethane foams containing an isocyanurate structure, such as alkali metal carboxylates, non-basic metal salts and tertiary amines, a sufficient effect can be obtained when the index is within a range of about 150 to about 300, but they are substantially ineffective when the index is within a range of 100 to 105. Accordingly, in the conventional methods, if it is necessary to change the index, it is also necessary to change the kind of catalyst used, and hence, various disadvantages are brought about in operations, such as mixing of the starting substances.